Testosterone therapy is currently indicated for treatment of male hypogonadism, anemia, breast cancer, and hereditary angioedema. It is also being considered for treating a variety of other conditions such as male osteoporosis that appear to be mediated by androgen deficiency.
Traditional modalities for administering testosterone have included: intramuscular injection of long-acting testosterone esters such as the enanthate because testosterone itself is rapidly degraded by the liver if administered orally; oral administration of testosterone undecanoate, which provides systemically available testosterone; and subcutaneous implantation of fused testosterone pellets. None of these traditional modalities provides totally physiological levels or circadian patterns of testosterone and its active metabolites, dihydrotestosterone (DHT) and estradiol (E.sub.2).
It is known that steroids, including testosterone, are absorbed through skin. However, the permeability to testosterone of skin areas that are normally used for transdermal delivery (e.g., the neck, back, chest, arms) is too low to permit delivery of the amounts of testosterone needed for therapy (typically 5-10 mg/day) through a limited area of skin. In this regard, Korennan, S. G., et al., (Am J Med (1987) 83:471-478) in an article on testosterone delivery for treating hypogonadism concluded "a more permeable skin area with a much higher absorption rate was required to provide programmed transdermal delivery to a limited area." This led Korennan et al. to select scrotal skin--which is highly permeable to testosterone--as a site for testosterone delivery. The article further describes a transscrotal delivery system developed by ALZA Corporation. U.S. Pat. No. 4,704,282 describes that system in detail. It consists of a polymer matrix that contains testosterone at subsaturation levels and a fabric reinforcement incorporated into the matrix that also is a limited solvent for testosterone. The patent indicates that a subsaturated matrix is used because a declining testosterone release rate is desired. The reinforcing fabric, in addition to providing a structural support function, is said to act as a secondary reservoir for testosterone which has the effect of flattening the release rate profile (see FIG. 2 of the patent). While the patent states that permeation enhancers may be present in the matrix, no examples of the use of such enhancers are described. The patent gives no data on the skin flux of testosterone provided by its systems. Example 2 of the patent states that its system may be applied to nonscrotal skin, particularly the thigh, to produce "similar results" as when applied to scrotal skin. This statement is, however, contradicted by the later Korennan et al. article (which also originates from ALZA Corporation) which reports that systems applied to the thigh did not give increased blood levels of testosterone.
Ahmed, S. R., et al. (J Clin Endocrinol Metab (1988) 66:546-557) and Findlay, J. C. (J Clin Endocrinol Metab (1989) 68:369-373) report that the 60 cm.sup.2 ALZA transscrotal system delivers about 3.7 mg/day and produces low-normal testosterone levels in hypogonadal men. Such dosages are believed to be somewhat less than the amount needed to mimic endogenous production (5-10 mg/day). Furthermore, since scrotal skin has a relatively high level of 5.alpha.-reductase, continuous transscrotal delivery of testosterone produces levels of DHT and DHT/testosterone ratios 4- to 5-fold greater than normal. Such abnormal levels and ratios may give rise to undesirable side effects.
In sum, the art teaches away from transdermally administering testosterone through nonscrotal skin because of the low permeability of such skin to testosterone. Transscrotal delivery of testosterone is taught, but such delivery is associated with high DHT and DHT/testosterone ratio levels and does not provide a level of testosterone delivery that mimics endogenous production. Further, scrotal skin is sensitive and limited in area, which may result in discomfort and poor patient acceptance of this modality of delivery.